The present invention relates to an optical fiber cable comprising an optical unit and a plurality of gel layers surrounding the optical unit. In particular, the present invention relates to an optical fiber cable comprising a buffer tube housing an optical unit and a plurality of gel layers having different rheological properties surrounding an optical fiber.
Optical fiber cables are used to transmit information at very high rates over long distances. Optical fiber cables may be classified into three general classifications based on cable structure: loose tube, monotube, and slotted core. In loose tube and monotube cables, buffer tubes are utilized as the primary structure for protecting the thin optical fibers contained within. In particular, the buffer tubes typically house an optical unit such as one or more loose optical fibers or an optical fiber ribbon stack having a plurality of optical fibers held together in a planar array by a common matrix material.
In a loose tube cable, a plurality of buffer tubes are stranded, helically or reverse helically, around a central strength member to form a stranded core. In addition to the buffer tubes, filler rods may be stranded around the central strength member in order to provide symmetry in design for fiber counts lower than that of a full fiber count cable. In a monotube cable, one or more optical fibers are housed in a single, centrally located buffer tube. Typically, the buffer tubes or stranded core is jacketed with an additional protective layer. Further, reinforcing yarns or fibers as well as waterblocking materials in the form of gels or hot melts, water swellable powers, yarns or tapes, and/or corrugated armor may be place between the jacket and the inner cable layers.
In a slotted core cable, the optical fibers reside in channels or slots which are generally filled with a gel material. These channels form a helical path along the longitudinal axis of the cable.
The buffer tubes"" primary function is protect the delicate optical fibers housed therein. Accordingly, control of the modulus, percent elongation to break, coefficient of thermal expansion, shrinkage, swelling and other physical properties of the buffer tubes is very important. Buffer tubes are typically made from xe2x80x9cengineering resinsxe2x80x9d such as polybutylene terepthalate (PBT), polycarbonate (PC), polyarnides such as nylon-12, polyolefin materials such as polyethylene-polypropylene copolymers and isotactic polypropylene (I-PP), or some layer combination of the above. See U.S. Pat. No. 6,085,009.
In the cable industry, it is well known that changes in ambient conditions lead to differences in water vapor pressure between the inside and the outside of a plastic cable jacket. This generally operates to diffuse moisture in a unidirectional manner from the outside of the cable to the inside of the cable. Eventually, this will lead to an undesirably high moisture level within the cable, especially if a plastic jacket is the only barrier to the ingress of the moisture. Water may also enter the cable because of rodent attacks or mechanical impacts that cause openings in the sheath system. While the presence of water within an optical fiber cable may not immediately impact its performance, the passage of the water along the cable interior to connection points or terminals or associated equipment inside closures, for example, may cause problems, especially in freezing environments and should be prevented.
The buffer tubes may be filled with a water blocking compound such as a thixotropic gel to prevent water ingress while allowing fiber movement during cable expansion or contraction. It is also know to use water swellable or superabsorbent materials, such as tape, power or yarn formed of polyacrylates with carboxylate functional groups, partially neutralized polyacrylic acid, polyarnides, or copolymers thereof, which can absorb water in the buffer tubes. Further, it is known to use a secondary low molecular weight oil to pre-wet the optical fibers and optical fiber ribbons in order to prevent water migration down interstices through the optical unit by way of capillary action.
Conventional buffer tubes are typically manufactured with a single thixotropic gel surrounding the optical unit. The yield stress of the gel allows the optical unit to freely move within the buffer tube so that the optical fiber may drift to locations other than the axial center of the buffer tube. However, it is important to prevent the optical unit from contacting the buffer tube wall which may result attenuation problems due to microbending and high stress.
The thixotropic gel material generally includes a thickening or gelling agent in a liquid carrier. Traditionally, three types of gels materials have been used in fiber-optic cables: (1) gels based on polar oils such as polyols; (2) gels based on natural or synthetic hydrocarbon oils; and (3) gels based on silicone oils. Organic and non-organic thickeners are typically present as relatively isometric colloidal particles. Gelling agents form a physical network structure in which the polymeric base oil molecules interact with the gelling agent through entanglements, adsorption onto the surface of particles such as pyrogenic silica, and/or some other secondary interaction. When a low stress is applied to a gel-like composition, the material acts substantially as a solid. If the stress is above a critical value (commonly known as the yield-stress of the material), then the secondary interactions are disturbed, the material flows, and as shear rate increases, viscosity decreases rapidly (i.e., materials having such characteristics are called xe2x80x9cthixotropicxe2x80x9d). This decrease in viscosity is largely reversible because it is typically caused by the temporary breakdown of secondary interactions between gelling agents and polymeric base oils. These interactions can reform following the release of shearing forces on the material.
The selection of gel materials is an important consideration in buffer tube manufacture since gel materials which are compatible with the material of the buffer tube may not be compatible with or protective of the optical unit it surrounds. Non-compatible gel materials can swell buffer tube polymers and are able to extract the additives therein and reduce the thermo-oxidative stability of the buffer tubes. U.S. Pat. No. 6,085,009 discloses a water blocking gel which is compatible with polyefin buffer tubes and is made of a polyolefin oil, wherein only a very small fraction of the polyolefin species have a molecular weight below about 2000.
The present invention is adapted to provide an optical fiber cable including a buffer tube structure wherein the optical unit is located in the buffer tube and protected from contact with an inner wall of the buffer tube. According to the present invention, there is provided an optical fiber cable comprising a buffer tube housing an optical unit including at least one optical fiber, and at least first and second gel layers interposed between the buffer tube and the optical unit, wherein the first and second gel layers have different rheological properties. The first gel layer surrounds the optical unit and the second gel layer surrounds the first gel layer.
In accordance with a preferred embodiment of the present invention, the inner gel layer may have a yield stress and a viscosity which are lower than a yield stress and a viscosity of the outer gel layer. The lower yield stress and viscosity of the inner gel layer serve to maintain the optical unit in an axial center position within the buffer tube and facilitate easy re-positioning of the optical unit to the axial center position when the buffer tube is flexed or bent. As a result, the optical unit may be maintained in a low stress state and stress-induced attenuation may be prevented.
The above and other features of the invention including various and novel details of construction and process steps will now be more particularly described with reference to the accompanying drawings and pointed out in the claims. It will be understood that the particular optical fiber cable embodying the invention is shown by way of illustration only and not as a limitation of the invention. The principles and features of this invention may be employed in varied and numerous embodiments without departing from the scope of the invention.